1. Field of the Invention
The present invention relates to an apparatus and a method for performing a heat treatment of a substrate, for example, a baking treatment of a mask substrate for a photomask after it is coated with a resist solution, or after it is exposed and before it is developed.
2. Description of the Related Art
In a semiconductor device fabricating process, a technique called photolithography is employed to form a resist pattern on a surface of a substrate for the semiconductor device. The photolithography technique is also employed for making a rectangular mask (reticle) used for an exposing step in the semiconductor device fabricating process. In making the mask, a substrate for the mask is coated with a resist solution to form a resist film on the substrate, the resist film is exposed to provide the resist film with a predetermined pattern, and the resist film is developed.
The resist solution comprises chemical components of the resist film dissolved in a solvent. After the substrate is coated with the resist solution, the substrate is subjected to a heat treatment called “baking,” thereby the solvent contained in the resist solution is vaporized to be removed. The baked substrate is subjected to a cooling treatment by which the temperature of the substrate is lowered to a predetermined temperature before exposure of the substrate. The heat treatment or the cooling treatment is carried out by placing a substrate on a table or a plate provided therein with a heating or cooling means.
FIG. 12 shows a conventional heat treatment apparatus. Reference numeral 1 designates a hot plate on which a substrate G is placed. Four projections 11 are arranged on an upper surface of the hot plate 1. The substrate G is supported by the projections 11 while the back surface of the substrate G is vertically spaced at a slight distance such as 0.5 millimeters from the upper surface of the hot plate 1.
A heating element 12 is arranged in the hot plate 1. The electric power supplied to the heater 12 is controlled by a PID (proportional integral differential) controller 14 based on a detection signal of a temperature sensor 13 such as a thermocouple, so that the temperature of the substrate G is heated at a predetermined target temperature such as 130° C. In order to treat plural substrates while achieving high throughput, the hot plate 1 is heated at 130° C. beforehand, and then the substrate G is placed on the hot plate 1. After the substrate has been heated for a predetermined time at the target temperature, the substrate is removed from the heat plate and the next substrate is placed on the hot plate 1. In this way, plural substrates are heat-treated one by one.
The temperature of the substrate G before it is placed on the hot plate 1 is about 23° C., for example, and thus a large temperature difference exists between the substrate G and the hot plate 1. Accordingly, when such a cold substrate G is placed on the hot plate 1, a considerably large amount of heat is transferred from the hot plate 1 to the substrate G, and the temperature of the hot plate 1 is thus considerably lowered.
If high PID control gains are set in the PID controller 14, the temperature of the hot plate 1 rapidly rises up to the original temperature, or the target temperature. However, in this case, the temperature of the hot plate 1 overshoots the target temperature and oscillates with high amplitude, and resultantly, the substrate temperature overshoots the target temperature. This causes undesirable variation in the thickness of the film (e.g., resist film) over the substrate surface.
In order to avoid such disadvantage, with the conventional heat treatment apparatus, the PID control gains are set low to raise the temperature of hot plate 1 slowly, as shown in FIG. 13, in order to prevent overshooting of the temperature of the hot plate 1. However, in this case, a considerably long time (TA) is required to return the temperature of the hot plate 1 back to the target temperature.
JP11-74187A discloses a heat treatment apparatus designed for solving the aforementioned problem. In the heat treatment apparatus, the temperature controller has two operation modes, one being a PID control mode in which the temperature is controlled by a PID controller, the other being a fixed power supply mode in which a large fixed electric power is supplied to a heating element.
Immediately after the cold substrate is placed on the hot plate, a fixed, large electric power (full power) is supplied to the heating element for a predetermined time by using a timer in order to rapidly replenish an amount of heat to be drawn from the hot plate by the substrate. The fixed electric power and the predetermined time period are determined so that the temperature of the hot plate overshoots the target temperature when the predetermined time has elapsed. When the predetermined time has elapsed, the supply of the fixed large electric power is stopped, and the operation mode is switched from the fixed power supply mode to the PID control mode.
However, the technique disclosed in JP11-74187A has the following problem. That is, the temperature of the hot plate continues to rise for a while, even after the supply of the large fixed electric power is stopped, in fact. Accordingly, the actual temperature of the hot plate changes independently of a manipulation amount calculated by the PID controller and thus stable PID control is impossible, and this condition continues for a while. Accordingly, a considerably long time is required for the temperature of the hot plate and the substrate to be stable at the target temperature.